Episode 2: Light Fantastic! - Telescopes

This blog post is a transcript of Series 1, Episode 2 of my science podcast, Don't touch my Radium! All sources, references and recommended material can be found here.

Last week we looked at the habitability of our very own planet Earth, and learned about how rare and wonderful we are. But early philosophers would still look up to the stars in the sky and wonder if life could exist further afield. In order to even contemplate the existence of life on other planets you first need to establish that exoplanets are even a thing. There are very early indications of the tracking of the motion of the stars but exoplanets are not visible to the naked eye. The creation of the telescope enabled scientists to fully realise the extent of our place in the stars and begin to look beyond the limits of our solar system . So let's look at how this invention came about and how it allows us to explore the universe.

Light and the Electromagnetic Spectrum

To understand how telescopes work, we need to understand how we interact with our environment. We have a complicated visual system that allows us to use perception to interpret our surroundings by processing the light that is reflected by the objects around us. But what do we mean by light?

We tend to think of light as the brightness around us, as our ability to see in the dark. But in reality light is just energy, it is a form of radiation that runs along a spectrum of different wavelengths, with ‘visible’ light being only a small portion of the whole truth. The spectrum is a range of frequencies of electromagnetic radiation that are categorised by their wavelengths. This form of energy is emitted by stars and is able to travel through the vacuum of space at the speed of light until they interact with some matter in their path. Some of the energy will be absorbed, and some of it reflected. When an atom absorbs radiation, the electrons in the shell can gain energy allowing them to become ‘excited’. But electrons don’t want to be in an excited state and so they will ultimately return to their original lower energy state and the extra energy will be released, the type of radiative energy emitted will depend on the atom and amount of energy.

James Clerk Maxwell is a super famous physicist who unified electric and magnetic fields into his theory of electromagnetism. Two of the important quantities within his theory are permittivity and permeability of free space, permittivity relates to electric fields and permeability to magnetic fields. What he noticed was that these quantities had a significant relationship to the speed of light. This indicated that light must be em radiation that is carried by an oscillating electromagnetic field, it acts as a wave.

However, Einstein's photoelectric effect observes that when metals are bathed in light, negatively charged electrons gain energy and escape from the metal. The energy the electrons carry depends on the intensity of the light, in other words the amount of electromagnetic radiation being released depended on temperature. This isn’t how a wave would behave, but it is how a particle would. But how can light be a particle when Maxwell showed that it is a wave, well this is where we say that light has wave-particle duality and as neither theory appears to be incorrect or that either has more evidence than the other. Both must be true. Einstein determined that light should be thought of as being carried in packets, with each packet having a ‘discrete energy’ value that is related to the wavelength of the light. These packets or as Planck said ‘quanta’ of energy are what we refer to as photons. Photons are fundamental particles that are carriers of energy with the amount of energy being determined by the type of radiation being propagated through space.

So electromagnetic waves are oscillating electric and magnetic fields that are made up of a stream of photons, which travel in a harmonic pattern at the speed of light through space.

The different wavelengths in the em spectrum are split into seven categories: radio waves, microwaves, infrared, ultraviolet light, x-rays, gamma rays as well as the visible light that we see. However, in terms of physics, there is no difference in how we mathematically explain the different waves, they are all forms of radiation. Radio waves have the lowest energy, gamma rays have the highest and visible light is in the middle.

As the nature of em waves is to carry information, such as energy and momentum, from their source and impart the details onto the matter it interacts with, we can find ways to read this information. We can tell a lot about an interstellar object from its colour. If you think about a burning fire, the flames change colour with temperature, the same goes for objects in our universe. Hot objects, like suns, will emit shorter wavelengths of energy and change colour as they heat up over time so we can use this information to learn about the stages of a star's life. We also use visible light to detect gaps in the visible wavelengths of objects in space, which can give us information about their composition. How do we accomplish this?

In the internal mechanics of our eyes, light enters and the lens of our eye focuses that light, directing it to the parts of our retina that are able to convert it into neuronal signals. Essentially, they are acting as photon detectors that can interact with wavelengths of one region of the em spectrum that we refer to as the visible region. Telescopes take this principle and use it to focus the light from distant objects so that we can observe them. The addition of spectrographs and detectors allows us to interpret wavelengths that we cannot see in order to learn more about objects both inside and outside our solar system.

Telescopes

Some of the earliest common uses of lenses were reading stones that magnified the letters making them easier to see. Over time the magnifying properties were manipulated to create the first eyeglasses in around the 13th century.

A simple object developed by a dutch glass maker who observed the magnifying properties of two lenses together led to the first patent for a telescopic lens and was originally used by seafarers to look out across the horizon for approaching land. The history of the development of the telescope is long, so very long! Over hundreds of years many scientists contributed to the technological development of the telescope from a handheld seafaring object to ground based observatories: multi-array sites: orbital satellites and travelling probes. We define a telescope as "an optical instrument designed using lenses and or mirrors to observe distant objects by their emission, absorption or reflection of electromagnetic radiation."

Spectroscopy

Refraction is the term used to describe the way the direction of light changes as it passes through two mediums of differing densities, there is a change in velocity due to the change in the medium, the angle of entry also causes a change in direction and as red light has a longer wavelength than blue, it has a lower angle of refraction, it bends less. The oldest example of spectroscopy was Newton’s prism - he allowed light to pass through a glass prism that refracts the light rays, as each ray of light was bent at a different angle, they exited the medium separately splitting the white light into all it’s different

colours/wavelengths. The first telescopes were based on the principle of refraction.

Refracting Telescope

A refracting telescope is made by combining two lenses, an objective lens to produce an image at its focus and an eyepiece lens to magnify the image. In this simple two lens system the image that is produced at the focal length is upside down, a third lens can be added that flips the image to upright.

A refracting telescope is often referred to as a Keplerian or Galilean telescope. This is due to their work in advancing the first version of telescopes to their use in astronomy. In about 1608, as the story goes, a dutch glassmaker saw some kids playing with some lenses in the light and got the idea for the first spyglass, used by sailors to spot land on the distance. Galileo, having heard about the design but never seeing it, devised his own version only a year later. In 1611, Kepler then advanced Galileo's design by replacing his concave lens with a convex one, most refracting telescopes now use convex lenses as this increased the field of view as well as the magnification.

Refracting telescopes are used to examine the visible-light region of the em spectrum. They work on the principle of magnification of objects. The magnification being determined by the distance between the objective and eyepiece, or more clearly, the ratio of the focal length of the objective to the focal length of the eyepiece. The objective should have the longer focal length and the eyepiece the shorter. Both lenses are convex so they bend the light inwards, which is why the image appears smaller, but closer allowing for observations of surface details. When observing planets and moons, magnification is important but stars are bright point sources at huge distances, so magnification doesn’t provide any useful information or advantage.

One of the biggest problems with refracting telescopes is chromatic aberration, the wavelengths bend like the prism as they pass through the lens producing a rainbow effect around the image. If you use a very long objective focal length you can reduce this effect, this is why early telescopes were so long! You could also try to use multiple lenses to try to compensate for the effect, or you can use mirrors! Light reflects the same way regardless of wavelength and so the use of mirrors instead of lenses solved the problems with chromatic aberration.

Reflecting Telescope

The reflecting telescope was designed by Newton, in about 1668, where he replaced the objective lens with a parabolic mirror to gather light in order to reduce the effects of chromatic aberration. Instead of refracting the light to a focus beyond a lens, it reflects the light back to a focus. When using a reflecting telescope, a larger mirror can gather a lot of light allowing you to view faint objects. This coupled with the magnification power improved the use of telescopes for viewing distant stars. Newton wasn’t the first to think of using mirrors, this was likely James Gregory in 1663, but Newton was the first to build one.

There are many different types of telescope as they will each operate in different wavelength bands and so can provide different information about the same object, modern day analysis will combine results to give a more comprehensive view of an object, such as the image of the crab nebula. These telescopes use methods of spectroscopy to analyse the em radiation released from the sun and other stars in the universe.

The colour spectrum that we see is produced when atoms are excited in ways that only emit certain colours. Sunlight contains all the colours and appears white, if you put it through a prism you will see a continuous color spectrum. However if you excite a gas with electricity it will only emit certain colours which appears as a black band with intermittent colour lines.

Newton would record the spectrum of sunlight by drawing it, when he created his reflecting telescope, he would do the same, look through the eyepiece and draw what he saw. However, as developments in photography came along, it became possible to photograph spectral lines onto a glass plate allowing for telescopes to be used to record the spectra of astronomical objects and comparisons to be made with spectra from other light sources with known wavelengths. Using spectroscopy to observe thousands of stars allowed early astronomers to identify characteristics in stars and create a spectral classification system.

Modern technological advances improved the functionality of telescopes to not only produce detailed images of distant objects, but also to connect with cameras, detectors and scientific instruments to collect and focus light and reproduce images along with spectral graphs. Photoelectric spectroscopy uses charged-coupled devices (ccds) that can convert more incident photons into information than using film/photographic plates. This also means that you can obtain spectra from fainter sources.

CCds are traditionally metal-oxide semiconductor sensors that were developed in the 60s by Willard Boyle and George E Smith, who both were awarded a nobel prize in 2009 for their work. We’ll talk about ccds in a bit more detail in a later episode when we cover the technology for space telescopes. For now I’m just going to give a little bit of background on the basic idea of a ccd and it’s application. A ccd acts as a photon detector by taking the principle of Einstein's photoelectric effect and moving electrical charge to an area of the device that can be converted into a digital signal. It is a tiny microchip that can focus the light collected by a telescope onto a large grid of pixels, when the light hits one of the pixels the electrons gain energy and are released. So the incoming photons are converted to electrical charges that can be read as digital signals in the form of the intensity of the light.

With developments in technology and scientific understanding, astronomical spectrographs started to use diffraction gratings to disperse the light. A diffraction grating works by reflecting the light so that no photons are lost through absorption in the medium. The spectrograph is designed to have a slit that will limit the light entering allowing it to act as a point source coming from a larger object. It means that you can take in spectra from different regions in the field of view of the telescope, the light is made parallel before it reaches the diffraction grating which disperses the light into its constituent wavelengths which can be focused by a camera mirror into a detector that can interpret the signals. Spectra recorded on CCds can then be digitized for storage and further analysis. This allows for faster processing and correction methods to be applied to the collected data. As a result, spectra is presented as a graph, a plot of intensity vs wavelength.

There are a number of ways to interpret these graphs, a continuum spectrum would just be a smooth curve depicting a dense hot object, maybe the core of a star. However, most stars will have gas in their outer layers that have a lower density than the core. When looking directly at the star, the photons that are emitted by the core are in all frequencies/energies. Specific energies can then be absorbed by the electrons in the outer layers causing a change in the electron energy levels. As it returns to its rest state, it emits a new photon of a specific frequency. The direction of this photon is random so that the intensity of light at this wavelength of the photon will be less in the direction of the observer. So the spectrum will show a dark line, or a decrease in the intensity as a dip in the graph referred to as an absorption line. If you are not looking directly at the body, you could be looking at a cloud of gas that is excited by the energy of the nearby star. Again, when the electrons return to rest state they emit photons of specific frequencies and wavelengths, emitting them in random directions so that the observer will record light as bright lines, that are termed emission lines.

If we can detect a spectral line that corresponds to a specific energy transition for an ion/element/molecule in a star's spectrum, this indicates its presence in the star. This is how we discovered helium in the Sun. So spectroscopy allows us to learn about the composition of the sun and this can be applied to other stars outside of our solar system.

Modern spectrographs have very high resolution and can show fine detail in spectral lines so that it’s possible to make some determinations about the relative amounts of different elements and molecules that are present, the relative composition of the star.

These methods can then be applied to the study and analysis of light emitted by exoplanets. Spectroscopy is the primary method used to hunt for biosignatures. Each element in the periodic table emits photons at certain, discrete, wavelengths in the excitation state of atoms. These appear as absorption lines in the spectrum of an astronomical object- by measuring the position of the spectral lines, we can determine which elements are present in the object or along its line of sight. Through this, we can learn about the types of molecules that are in the atmosphere.

Almost all of the major telescopes currently used in astronomical research are reflecting telescopes. Reflecting telescopes can examine the visible, infrared and uv wavelengths of the em spectrum. This is because the uv and infrared wavelengths would normally be absorbed when passing through glass lenses. They were originally made from metals but were subject to tarnishing so by the late 19th century the trend changed to using glass coated with thin layers of silver. This alteration set the path for reflecting telescopes to become more popular and increase their usage. As the light does not need to pass through them, they can be supported entirely on one side, this means that they are not subject to sagging and warping from their weight and so can be built in larger sizes. The largest refracting telescope is 1m where as the largest reflecting telescope is over 10 m.

As the design and size of telescopes progressed, they developed into observatories. Large building structures, traditionally a dome, that house and protect instruments with diameters of up to 10.4 meters such as the Keck Observatory in Hawaii. (ELT set to be ready in 2025 with diameter of 40m) Observatories on the ground are able to 'make observations' in the radio and visible light portions of the EM spectrum only. All the ranges of em radiation are released from the sun but our atmosphere acts as a barrier stopping some wavelengths from getting through to the surface.

• Radio : a portion gets through, even on a cloudy day

• Microwaves: most of this light is blocked

• Infrared: some can get through but longer wavelengths are blocked, also as everything with heat emits IR it can get mixed up, if you measure the IR of the atmosphere and keep the equipment cooled then you can calculate the amount coming in from an object.

• Visible light: right through, coined the term optical astronomy.

• UV absorbed in atmosphere

• X-rays: blocked by atmosphere

• Gamma rays: blocked, also really hard to focus on these wavelengths.Last week we talked about how the ozone layer blocks ultraviolet rays.

A modern observatory needs to be placed in an area that has dry air, a large amount of clear nights, a high elevation to reduce effects from atmospheric turbulence (when higher the atmosphere is thinner) and dark skies.

We as humans can see the wavelengths of the visible region, but in order to read and make determinations about the other parts we need to create detectors that are able to interact with this form of light. In terms of the hunt for biosignatures we mostly look at wavelengths that lie in the visible and infrared ranges. The visible range comes in at about 400nm-700nm (visible), with 700nm-5000nm(Near IR) and 5000nm-25000nm (Mid-IR). We don’t look at the far IR region as planets don’t emit in that range but it’s good for looking at space dust so it’s used for looking at other galaxies...i think.

Infrared light was discovered, actually by accident by William Herschel, of the Herschel space observatory . He was testing the temperature of the different waves of visible light and put a thermometer outside the region, just past the red light to use as a control, what actually happened was a reading higher than the ones on for the visible light. Infrared can be used to reveal objects in the universe that wouldn’t register on an optical telescope as planets absorb light from their sun and then radiate this heat outwards as infrared light. Once emitted the infrared wavelengths, being longer than the visible range, are able to pass through dense regions of dust and gases in space without losing a lot of light through scattering and absorption. So at mid-infrared ranges, cooler objects like planets can be viewed more clearly.

But as light travels through the vacuum of space until it interacts with matter once visible light meets our atmosphere, that is filled with molecules and gases, the different wavelengths pass through all these ‘mediums’ with different speeds and so the light rays get refracted before they reach the telescope and the different rays can’t come to a common focus and the image is blurred. This mostly happens close to the ground, in the 30 meters above the surface, this is why a lot of telescopes are mounted on mountains where the atmosphere is thinner, it also moves them away from the effects of light pollution.

As we move forward, what are the most important aspects of a telescope? Yes magnification is important but as mentioned earlier this does not work for distant objects. The way we determine the power of a modern telescope is by its light gathering ability and it’s resolving ability. We need large apertures to see faint objects that are far away as they can gather more light increasing the brightness of the object allowing it to be dispersed into a spectrum. The resolving power relates to the ability to see small detail and sharp images. It’s related to the angle of resolution, measured in arcseconds which is an indication of the telescopes ability to separate objects close together at a distance like maybe a star and a planet. We want the smallest angle of resolution as possible and this is achieved by making larger telescope apertures.

Modern telescopes do not produce perfect images. They have advanced a long way with ground based observatories overcoming many challenges and becoming capable of seeing further than before. However, if we really want to get a clearer image of what's out there, we need to get away from the light pollution and atmospheric effects. We need to go up! Next week we'll look at satellite technology as it relates to orbiting telescopes and what is required to send a probe out into space and how this improved the knowledge of our own solar system.

Articles

• What is a ray of light made of? , Colin Barras, 2015, BBC Earth

• What is Electromagnetic Radiation?, Jim Lucas, 2015, LIve Science

• "How Telescopes Work", Craig Freudenrich, 2000, HowStuffWorks.com

• Life probably exists beyond Earth. So how do we find it?, Jamie Shreeve, 2019, National Geographic

Lecture Notes / Education Sites

• Near, Mid & Far Infrared, University of Durham

• What is electromagnetic radiation?, Northwestern University

• Observatories Across the Electromagnetic Spectrum, Imagine the Universe, Goddard Space Flight Centre, NASA

• Spectroscopy, Swinburne University

• Telescope History, NASA Education

• Reflecting Newtonian telescope, University of New South Wales

• Reflecting Telescopes, Prof H. E. Smith, University of California San Diego

• Spectrographs, Robert Hollow, Australian Telescope Outreach and Education website, Australia Telescope National Facility

Documentaries / Youtube Videos

• The True Nature of Light and Energy, BBC documentary, YouTube

• Tour of the Electromagnetic Spectrum, ScienceAtNASA, YouTube

The Electromagnetic Spectrum. Image sourced from NASA Jet Propulsion Lab website, click on the image for a link. This image is available under Creative Commons Licence.